1. Field of the Invention
The present invention relates generally to a catalyst for purifying exhaust gas disposed in an exhaust system of an internal combustion engine. More particularly, the present invention relates to a diagnostic apparatus for controlling the operation of a catalytic converter.
2. Description of the Related Art
Generally, three way or reduction type catalytic converters are utilized in the exhaust systems of internal combustion engines for the purpose of reducing three types of toxic exhaust gas emissions: hydrocarbons, carbon monoxide and nitrogen oxides. These converters typically employ platinum, palladium and rhodium coated materials within the converter as catalysts that, once heated to a certain temperature, oxidize or reduce the exhaust emissions which pass through the converter. Car manufacturers often use two converters for an exhaust system: a first mini catalytic converter close to the engine that heats up quickly to reduce emissions during engine warmup, and a main converter placed behind the mini converter to oxidize larger volumes of exhaust emissions. Over a converter's operational lifetime, the desirable physical properties of catalytic materials that quickly heat up and function as efficient catalysts undergo a natural deterioration. With this deterioration, the efficiency of the converter likewise decreases. To counter this tendency, advances have been made to monitor and compensate for the deterioration of the catalytic materials. For example, Japanese Unexamined Patent Publication No. 1-216009 discloses a catalytic diagnostic apparatus which continuously monitors the condition of the catalytic material in order to compensate for the decrease of catalytic efficiency of the converter over time.
In the disclosed apparatus, a catalytic converter is disposed in an exhaust gas passage of an engine. A pair of thermocouples, acting as temperature detecting means, are disposed at the inlet and outlet ports of the catalytic converter. Output signals from the thermocouple are compared immediately after engine startup for an indication of the temperature differential at the inlet and outlet converter ports. In this way, a quantitative and indirect means is provided for detecting the temperature of the catalytic material and consequently, the relative effectiveness of the converter. When the effectiveness of the converter is sufficiently degraded, a malfunction warning apparatus is further provided to inform the automobile operator of the catalytic converter's decreased effectiveness.
However, using thermocouples as described above presents design and manufacturing disadvantages. Manufacturing thermocouple units designed to be placed in both inlet and outlet ports of catalytic converters results in a converter unit that is both bulky and prone to having manufacturing flaws. It is nonetheless advantageous to know the condition of the catalytic material, and thus, efforts continue to design and manufacture an apparatus which detects the condition of the catalytic material and which is simple in construction and design.